1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for determining a Q-factor of S-waves near a surface of the earth.
2. Discussion of the Background
During the past years, interest in monitoring oil and/or gas reserves has increased, Time-lapse (or 4D) seismic monitoring of producing oil fields is an accepted method for optimization of field development and product recovery, providing significant improvements in recovery rates and savings in drilling costs.
Time-lapse seismic reservoir monitoring is the comparison of 3D seismic surveys at two or more points in time. Time-lapse seismic reservoir monitoring also has potential for increasing ability to image fluid movement between wells. A traditional configuration for achieving a 4D seismic monitoring is illustrated in FIG. 1. FIG. 1 shows a system 10 for the acquisition of seismic data. The system 10 includes plural receivers 12 positioned over an area 12a of a subsurface to be explored and in contact with the surface 14 of the ground. A number of vibroseismic sources 16 are also placed on the surface 14 in an area 16a, in a vicinity of the area 12a of the receivers 12. A recording device 18 is connected to the plurality of receivers 12 and placed, for example, in a station-truck 20. Each source 16 may be composed of a variable number of vibrators, typically between 1 and 5, and may include a local controller 22. A central controller 24 may be present to coordinate the shooting times of the sources 16. A GPS system 26 may be used to time-correlate the sources 16 and the receivers 12.
With this configuration, sources 16 are controlled to generate seismic waves, and the plurality of receivers 12 record waves reflected by the oil and/or gas reservoirs and other structures. The seismic survey may be repeated at various time intervals, e.g., months or years apart, to determine changes in the reservoirs. Although repeatability of source and receiver locations is generally easier to achieve onshore, the variations caused by changes in near-surface can be significantly larger than reservoir fluid displacement, making time-lapse 4D seismic acquisition and repeatability challenging. Thus, variations in seismic velocity in the near-surface are a factor that impacts repeatability of 4D surveys. A secondary effect is the Q-factor. Thus, knowing the Q-factor helps in determining these changes in the near-surface.
As operators move to understand the behavior of the reservoir over time, particularly as it applies to mapping the migration paths of hydrocarbons and improving the performance of the reservoir under certain stimulation, 4D reservoir monitoring becomes increasingly more important for overall field development. However, to improve the calculations indicating the behavior of the reservoir based on 4D data, being able to accurately estimate the Q-factor is desirable.
Thus, there is a need for developing a device and a method for better estimating the effect the near-surface is having on the seismic waves (e.g., Q-factor) for improving the 4D calculations.